-LRB- CNN -RRB- -- The 15-year-old Mexican youth who was shot and killed by a Border Patrol agent had a history of involvement with human smuggling and was on a list of repeat juvenile offenders , U.S. Customs and Border Protection spokesman Mark Qualia told CNN Thursday .

The victim , Sergio Adrian Hernandez Guereca , had been apprehended by U.S. officials on more than one occassion but was never criminally charged , Qualia said .

The use of juveniles to smuggle people across the border is a common tactic for smugglers , he said .

Meanwhile , a video obtained by CNN casts doubt on the Border Patrol agent 's claim that he was surrounded by rock-throwing suspected illegal immigrants when he fatally shot the boy on the border at Ciudad Juarez .

CNN obtained the video , which was shot by a witness on a cell phone camera from the Mexican side of the border , from affiliate Univision . The video aired on its program Primer Impacto late Wednesday evening .

The video shows part of the buildup before the incident , with several individuals running underneath the Puente Negro , a railroad span that connects the two countries .

Monday night 's incident started around 6:30 p.m. when Border Patrol agents responded to a report of a group of suspected illegal immigrants being smuggled into the United States near the Paso del Norte port of entry , FBI Special Agent Andrea Simmons said .

In the distance , a U.S. Border Patrol officer on a bicycle can be seen making his way toward the area . Seconds later , the officer can be seen getting off the bicycle and approaching two of the four suspected Mexican nationals who had just crossed through an opening in the fence . One of the suspects is detained by the officer , but never handcuffed , and is dragged a short distance . This happened on the U.S. side of the border .

Moments later , the officer points what appears to be his firearm in the direction of a second suspect , standing about 60 feet away from the officer -- on the Mexican side of the border . The video shows the suspect running away .

Seconds later , two gunshots can be heard on the video . A third gunshot is heard in a different sequence of the tape . After the shooting , another suspect is seen running in the upper left side of screen away from the incident .

`` They 're throwing rocks , '' witnesses screaming in Spanish can be heard in the background of the video as the officer opens fire . `` They hit him ... they hit him . ''

The video contradicts Simmons ' account . She had said : `` This agent , who had the second subject detained on the ground , gave verbal commands to the remaining subjects to stop and retreat . However , the subjects surrounded the agent and continued to throw rocks at him . The agent then fired his service weapon several times , striking one subject who later died . ''

A federal law enforcement official told CNN that the FBI 's use of the word `` surrounded , '' was `` probably not the best choice of words , '' and that it is more accurate to say that people were nearby throwing rocks .

The FBI has been studying videos of the incident and said some of the video does show rocks being thrown at the Border Patrol agents , the official said .

Hernandez Guereca was a secondary student in Juarez .

`` The young man was not armed , '' said Sergio Belmonte , Ciudad Juarez spokesman . `` He did not have the physical size to threaten anyone . The aggression -LRB- by the U.S. agent -RRB- is evident . '' Belmonte said Hernandez was shot in the head .

`` My people have spoken to his family . His dad says he was a straight-A student . His secondary school even sent him on an academic trip because of his good grades , '' Mayor Jose Reyes Ferriz said .

A memorial for Hernandez was to be held in Juarez Thursday morning , according to Reyes . Local politicians were expected to attend .

Reports that the Mexican military may have drawn weapons on U.S. Border Patrol agents , which surfaced earlier Wednesday , could not be immediately confirmed . The tape released by Univision did not show any Mexican military troops .

`` We are aware of those reports , but I can not confirm them to you at this time , '' said Mexican military spokesman Enrique Torres . `` I plan on speaking with the individuals who are said to have been involved , but I ca n't and wo n't confirm that to you . I can not speculate . ''

The Mexican government has requested a quick and transparent investigation into the fatal shooting .

Mexico `` reiterates that the use of firearms to repel a rock attack represents a disproportionate use of force , particularly coming from authorities who receive specialized training on the matter , '' the Foreign Ministry said Tuesday in a news release .

Simmons told CNN earlier that she did not know whether the person who was shot was on the Mexican or U.S. side of the border , but the agent never left U.S. territory .

The body was found on the Mexican side of the border , Simmons said .

Peaceful protests against the U.S. Border Patrol were held in Juarez on Wednesday , near the site of the fatal shooting .

In video shot by CNN affiliate KVIA , a man holding a bullhorn is seen pacing back and forth on the dusty streets of Juarez , pleading `` Justice for Hernandez '' to any passerby who would listen .

The shooting comes less than two weeks after the May 31 death of a Mexican illegal immigrant who had been detained three days earlier by border agents in California .

A suspect identified as Oscar Ivan Pineda Ayala was initially detained on the Rio Grande levee , said the FBI , which is leading the investigation .

`` The growing frequency of this type of event reflects a worrisome increment in the use of excessive force on the part of some border authorities , '' the Mexican Foreign Ministry said .

According to the ministry , the number of Mexicans who have been killed or wounded by U.S. border authorities has increased from five in 2008 to 12 in 2009 and 17 so far this year .

Earlier , Qualia , the U.S. Customs and Border Protection spokesman , said he could not comment because he does not know where the Mexican government obtained its statistics .

But Qualia said there were 799 assaults on border agents from October 1 , 2009 , through May 31 -- up from 745 assaults for the same time period in 2007-08 and 658 for the same span in 2008-09 .

Lethal force , he said , is allowed `` when an agent is in imminent threat of physical or bodily harm , which could cause death or injury or in protection of an innocent third party . ''

The determination of when to use lethal force , Qualia said , is made by each individual agent at the scene .

From October 1 through May 31 , he said , Custom and Border Protection agents used their firearms 31 times .

Rock-throwing can be considered a dangerous assault , Qualia said : `` They 're not chunking pebbles . ''

CNN 's Nick Valencia , Arthur Brice , Jeanne Meserve and Devon Sayers contributed to this report .

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NEW : The Mexican teen killed in Border Patrol incident had been detained before , agency says

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Cell phone video sheds light on shooting death of Mexican teen by U.S. Border patrol agent

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Video does n't show agent surrounded by rock-throwing people as he shoots

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The Mexican government calls use of gun against rock throwers `` disproportionate ''